I don't think bandwidth is the issue. The link mostly handles low bandwidth traffic like rdp and ssh but it does handle a good deal of that traffic. It could be a packet per second issue but think the HP 4018GL is rated 72 million pps. If that was the issue, I think I would see the issues going from switch A to switch B w/o routing and I only see it from switch A to B when it is first routed. Could the routing be handled differently if the traffic doesn't originate from the switch? That seems like a long shot but I plan to do some testing to try and verify that but have not had the time yet. I also plan to test a secondary router when they let me.
More pieces to the puzzle...
switch A port A1 counters - Up 9 days.
switch B port A1 counters